Power consumed by a conventional Read Only Memory (ROM) device is data dependent. Data stored in a low voltage state consumes more power than data stored in a high voltage state due to increased bitline toggling. The toggling occurs when a pre-charged bitline is discharged while reading low voltage type data and then pre-charged again as part of a subsequent read.
Unused memory locations in the ROM are often padded to high voltage type values such that when addressed, the corresponding bitlines do not discharge. As such, the power consumption in the ROM will be low when addressing the unused locations. However, the padding only considers the unused address spaces. Padding offers no benefits to the overall power consumed when reading the full ROM.